1. Field of the Invention
The present invention relates to a manufacturing method of an integral type crankshaft bearing cap for an internal combustion engine. The integral type crankshaft bearing cap is integrated by spacing a plurality of crankshaft bearing caps along a beam to which they are connected.
2. Description of the Prior Art
In internal combustion engine technology, crankshaft bearing caps have been integrated into an integral structure. In the integral type crankshaft bearing cap, a plurality of crankshaft bearing caps are spaced in a longitudinal direction of the engine along a beam extending in the longitudinal direction of the engine to which the caps are connected. Since the integral type crankshaft bearing cap has high rigidity as a whole, rigidity of a cylinder block, to which the integral type crankshaft bearing cap is fixed, is increased, and thereby the level of vibration and noise of the engine is decreased. Two integral type crankshaft bearing cap structures are known. Either the crankshaft bearing caps and the beam are integrally formed using the same material, or the crankshaft bearing caps and the beam are constructed of different kinds of metal.
FIG. 1 shows an example of the former type of integral type crankshaft bearing cap. Crankshaft bearing caps 1 and beam 2 are formed integrally in casting, with crankshaft bearing caps 1 and beam 2 constructed of the same metal. With integral type crankshaft bearing cap 3, if the shape of crankshaft bearing caps 1 is intricate, molds for casting also become intricate and the molds must include a number of split molds. For instance, when recessed portions are formed on the wall of the crankshaft bearing caps to decrease their weight, the molds must be divided into a number of split molds.
However, as the number of split molds increases, the productivity of the integral type crankshaft bearing cap manufacturing process decreases. To raise productivity, it is almost unavoidable to form the portions for the crankshaft bearing caps in the shape as shown in FIG. 1 so as to be able to eliminate molds after casting; that is, crankshaft bearing caps 1 have flat walls and they are formed as plate-like blocks having almost uniform thickness. As a result, crankshaft bearing caps 1 have unnecessary metal with respect to strength, increasing the weight of integral type crankshaft bearing cap 3.
FIG. 2 shows an example of the second type of integral type crankshaft bearing cap using different metals. Such a structure is disclosed, for example, in Japanese Utility Model Publication No. SHO 57-112056. Integral type crankshaft bearing cap 4 in FIG. 2 is constructed of crankshaft bearing caps 5 and two beams 6, and manufactured as follows. Two beams 6 are set in a casting mold defining cavities therein, the cavities being formed in the same shape as crankshaft bearing caps 5. After that, molten metal is poured into the cavities of the mold, forming crankshaft bearing caps 5 around beams 6. In such a manufacturing process, since beams 6 must be set in a mold before casting, it is difficult to divide the mold into a number of split molds. Therefore, the possible shapes for crankshaft bearing caps are restricted, and it becomes difficult to form crankshaft bearing caps in a shape having recessed portions in their walls. As a result, as shown in FIG. 2, crankshaft bearing caps 5 are also formed as flat wall type blocks, with unnecessary portions increasing their weight.
Thus, in conventional structures made by conventional manufacturing processes, it is difficult to decrease weight by complicating the shape of the crankshaft bearing caps while maintaining an acceptible level of productivity.